SEA SURFACE TEMPERATURE RECONSTRUCTIONS OF THE WESTERN INTERIOR SEAWAY THROUGH SPACE AND TIME FROM BACULITES SP. SHELL MATERIAL

The paleoceanography and sea surface temperatures (SST) of the Cretaceous Western Interior Seaway (WIS) remain uncertain both spatially and temporally. Reconstruction of the variability in SST gives paleo-researchers insight into the past climate, oceanography, and ecological habitats at that time. In this study, we analyze a suite of extinct externally-shelled cephalopods that were ubiquitous in the WIS: baculitid ammonites. These cosmopolitan organisms grew their aragonitic shells via accretion, thus recording the paleo-SST of their environment in the oxygen stable isotopes of their shells. In this preliminary dataset, we use specimens of Baculites sp. from USGS paleontology collections, recently moved to the Smithsonian National Museum of Natural History. The specimens represent ~17 ammonite taxon range zones of the WIS. We drilled samples from each specimen for light stable isotope analysis. To support the validity of our stable isotope measurements with confirmed original shell material, we analyze flakes from each specimen using Scanning Electron Microscopy (SEM) and assign a shell quality preservation index to every baculitid studied. This preliminary suite of Baculites sp. was selected because they represent the largest latitudinal range of well-preserved specimens within our available samples.

Preliminary results of Baculites sp. from the Lower Maastrichtian (~69.5-70.1 Ma, n=6) show no correlation between SST from oxygen isotopes and latitude. Over a range of 7.5° of latitude, there is a range of ~1‰, or ~4°C variation in SST, versus a ~5-6°C temperature gradient across this latitudinal range from climate models. Two specimens of the same species from the same locality yield δ¹⁸O values indicative of ~2.8°C difference, suggesting varied qualities of preservation or different seasonal period of growth through which was sampled (i.e., winter vs. summer SST). Analysis of additional specimens is needed to confirm trends in SST in the Lower Maastrichtian.

Future work will involve analysis of additional ammonoid morphotypes, as well as benthic mollusks from the same collections. These will provide additional constraints and points of comparisons of ocean temperatures in different environments of the seaway through more of the Late Cretaceous (n ≈ 80).